The conversion of gas streams into liquid products by contact with a conversion medium in a liquid phase is well practiced in many fields. Where the solubility of the gas stream is limited, contacting and conversion of the gas stream of requires that the gas stream be disbursed within a liquid medium as a fine dispersion of micro bubbles to increase the mass transfer between the gas phase and the liquid phase. Getting the gas into the liquid phase is energy intensive and ways are continually sought to reduce the expense of providing the necessary energy to create a two phase dispersion of the gas and liquid.
A wide variety of devices are known for the dispersion of gas and liquid medium. Such devices include venturi injectors, slot injectors, or jet injectors and other high pressure mixers. Such gas transfer devices have found widespread use in a variety of fields including those of wastewater treatment and fermentation.
The field of fermentation is one in which particular application of this is of interest due to the increased emphasis on the conversion of renewable sources into liquid products such as motor fuels
Biofuels production for use as liquid motor fuels or for blending with conventional gasoline or diesel motor fuels is increasing worldwide. Such biofuels include, for example, ethanol and n-butanol. One of the major drivers for biofuels is their derivation from renewable resources by fermentation and bioprocess technology. Conventionally, biofuels are made from readily fermentable carbohydrates such as sugars and starches. For example, the two primary agricultural crops that are used for conventional bioethanol production are sugarcane (Brazil and other tropical countries) and corn or maize (U.S. and other temperate countries). The availability of agricultural feed stocks that provide readily fermentable carbohydrates is limited because of competition with food and feed production, arable land usage, water availability, and other factors. Consequently, lignocellulosic feed stocks such as forest residues, trees from plantations, straws, grasses and other agricultural residues may become viable feed stocks for biofuel production. However, the very heterogeneous nature of lignocellulosic materials that enables them to provide the mechanical support structure of the plants and trees makes them inherently recalcitrant to bioconversion. Also, these materials predominantly contain three separate classes of components as building blocks: cellulose (C6 sugar polymers), hemicellulose (various C5 and C6 sugar polymers), and lignin (aromatic and ether linked hetero polymers). For example, breaking down these recalcitrant structures to provide fermentable sugars for bioconversion to ethanol typically requires pretreatment steps together with chemical/enzymatic hydrolysis. Furthermore, conventional yeasts are unable to ferment the C5 sugars to ethanol and lignin components are completely unfermentable by such organisms. Often lignin accounts for 25 to 30% of the mass content and 35 to 45% of the chemical energy content of lignocellulosic biomass. For all of these reasons, processes based on a pretreatment/hydrolysis/fermentation path for conversion of lignocellulose biomass to ethanol, for example, are inherently difficult and often uneconomical multi-step and multi conversion processes.
An alternative technology path is to convert lignocellulosic biomass to syngas (also known as synthesis gas, primarily a mix of CO, H2 and CO2 with other components such as CH4, N2, NH3, H2S and other trace gases) and then ferment this gas with anaerobic microorganisms to produce biofuels such as ethanol, n-butanol or chemicals such as acetic acid, butyric acid and the like. This path can be inherently more efficient than the pretreatment/hydrolysis/fermentation path because the gasification step can convert all of the components to syngas with good efficiency (e.g., greater than 75%), and some strains of anaerobic microorganisms can convert syngas to ethanol, n-butanol or other chemicals with high (e.g., greater than 90% of theoretical) efficiency. Moreover, syngas can be made from many other carbonaceous feedstocks such as natural gas, reformed gas, biogas, peat, petroleum coke, coal, solid waste and land fill gas, making this a more universal technology path.
However, this technology path requires that the syngas components CO and H2 be efficiently and economically dissolved in the aqueous medium and transferred to anaerobic microorganisms that convert them to the desired products. And very large quantities of these gases are required. For example, the theoretical equations for CO or H2 to ethanol are:6CO+3H2O→C2H5OH+4CO2 6H2+2CO2→C2H5OH+3H2OThus 6 moles of relatively insoluble gases such as CO or H2 have to transfer to an aqueous medium for each mole of ethanol. Other products such as acetic acid and n-butanol have similar large stoichiometric requirements for the gases.
Furthermore, the anaerobic microorganisms that bring about these bioconversions generate very little metabolic energy from these bioconversions. Consequently they grow very slowly and often continue the conversions during the non-growth phase of their life cycle to gain metabolic energy for their maintenance. Many devices and equipment are used for gas transfer to microorganisms in fermentation and waste treatment applications. These numerous bioreactors all suffer from various drawbacks. In most of these conventional bioreactors and system, agitators with specialized blades or configurations are used. In some others such as gas lift or fluidized beds, liquids or gases are circulated via contacting devices. The agitated vessels require a lot of mechanical power often in the range of 4 to 10 KW per 1000 liters—uneconomical and unwieldy for large scale fermentations that will be required for such syngas bioconversions. The fluidized or fluid circulating system cannot economically provide the required gas dissolution rates. Furthermore, most of these reactors in the process are configured for use with microorganisms in planktonic or suspended form i.e. they exist as individual cells in liquid medium.
In the field of fermentation the use of gas injection devices is known to disperse gas streams into liquids. U.S. Pat. No. 4,426,450 discloses a fermentation vessel that uses a plurality of jet injectors to mix air and a fermentation broth in the bottom of a fermentation vessel. The '450 reference requires a gas stream at sufficient pressure to overcome the hydraulic pressure of the liquid in the vessel.
Furthermore, for the suspended cultures to get high yields and production rates the cell concentrations in the bioreactor need to be high and this requires some form of cell recycle or retention. Conventionally, this is achieved by filtration of the fermentation broth through microporous or nonporous membranes, returning the cells and purging the excess. These systems are expensive and require extensive maintenance and cleaning of the membranes to maintain the fluxes and other performance parameters. Cell retention by formation of biofilms is a very good and often inexpensive way to increase the density of microorganisms in bioreactors. This requires a solid matrix with large surface area for the cells to colonize and form a biofilm that contains the metabolizing cells in a matrix of biopolymers that the cells generate. Trickle bed and some fluidized bed bioreactors make use of biofilms to retain microbial cells on solid surfaces while providing dissolved gases in the liquid by flow past the solid matrix. They suffer from either being very large or unable to provide sufficient gas dissolution rates.
Moving Bed Biofilm Reactors (MBBR) have been shown to be high-rate, compact system for wastewater treatment, particularly where slow growing organisms are involved. Hallvard, Odegaard describes the use of MBBR system for the treatment of wastewater in Innovations in wastewater treatment: the moving bed biofilm process—Water and Science & Technology Vol 53 No 9 pp 17-32. These biofilm type rectors are especially compatible with highly efficient (in terms of both gas transfer efficiency [power per mass of gas transferred] and dissolution efficiency) such as jet and/or slot aerators/gas transfer devices. The combination of the MBBR process and these gas transfer devices overcomes the problems associate with alternate approaches described above.
It is also highly desirable to retain the microorganisms in the form of a biofilm. It is known that single organism systems are susceptible to phase attack. However, forming a biofilm is one known method to reduce susceptibility of microorganisms to a phage attack.